Scouring is an erosion process that occurs due to the movement of water. As water flows forcefully it can remove sediment and rock, creating a channel or pool. Scouring is also often accelerated due to dredging. Some tidal scouring can be attributed to the formation of features such as the San Francisco Bay.
Scouring also plays an important role after the release of major ice jams. When an ice jam is removed, there is a quick acceleration of water over a specific area. This can erode large amounts of sediment and rock. Here is a technical article on a specific event in 1984.